Carbonates such as ethylene carbonate, propylene carbonate and dimethyl carbonate are used as a solvent for an electrolyte salt of lithium secondary battery, solar cell, radical battery and capacitor. However since a flash point is low and combustibility is high, there is a danger of firing and explosion attributable to over-charging and over-heating, and in addition, since viscosity is high and conductivity at low temperatures is low, there is a problem that an output is decreased.
In addition, in lithium batteries, increase of a withstand voltage of an electrolytic solution is demanded for increasing a capacity. Further in capacitors, it is desirable that both of a negative electrode and a positive electrode are made of hard carbon, particularly it is desirable that a capacitor can be used stably at a voltage of not less than 3 V. In the case of solvents for electrolyte salts such as propylene carbonate and dimethyl carbonate which are conventionally used, decomposition of an electrolytic solution occurs at a voltage of not less than 3 V, and therefore such solvents cannot be used.
To solve the mentioned problem, methods of adding a fluorine-containing ether compound have been proposed (JP6-176768A, JP8-37024A, JP11-307123A, JP2000-294281A, etc.).
However fluorine-containing ether compounds have disadvantages that solubility of a lithium salt which is an electrolyte salt is low, and since compatibility of the compounds with hydrocarbon carbonates which are used as a solvent for dissolving an electrolyte salt is low, separation into two phases occurs.
On the other hand, use of a fluorine-containing chain carbonate which is obtained by fluorinating a chain carbonate is proposed. For example, there are proposed a carbonate having a fluorine-containing alkyl group at one end and a hydrocarbon alkyl group at another end (JP6-219992A), carbonates in which an end of a fluorine-containing alkyl group is perfluoro, such as a dicarbonate having ethylene in the midst thereof and CF3(CF2)(CH2)— at both ends thereof (JP10-149840A) and a carbonate having hexafluoroisopropyl groups at both ends thereof (JP2001-256983A), and in addition, a carbonate having a fluorine-containing ether group at one end and a hydrocarbon alkyl group at another end (JP2000-327634A).
However the carbonates in which an end of fluorine-containing alkyl group is a perfluoro group (JP10-149840A and JP2001-256983A) have disadvantages that solubility of an electrolyte salt is low and compatibility with a hydrocarbon solvent is poor. In the case of carbonates having a hydrocarbon alkyl group at one end (JP6-219992A and JP2000-327634A), solubility of an electrolyte salt and compatibility with a hydrocarbon solvent are improved, but lowering of flame retardance and withstand voltage due to decrease in a fluorine content is found.
In the case of electrolytic solutions for capacitors and radical batteries which repeat charging and discharging similarly to lithium secondary batteries, there are desired enhancement of flame retardance and withstand voltage and enhancement of low temperature characteristics in which viscosity is not high even at low temperatures and yet decrease in conductivity is small.